As long as the optical components in the system are larger than about 10x the System Wavelengths and accurate dispersion data is available, OpticStudio can be used to model propagation at any part of the electromagnetic spectrum.

That said, the accuracy of using rays to simulate the interaction of electromagnetic (EM) radiation with optical components is based upon a few assumptions. As long as these assumptions are not violated, the exact wavelength used for the simulation is arbitrary.

1. A ray represents some finite portion of a wavefront. A ray itself is infinitely thin; it has no associated area. This ray approximation is valid when the transverse dimensions of the optical elements through which the ray is propagating are at least several times the wavelength of the ray. When the element size begins to approach the wavelength, the wave nature of light begins to dominate and rays will not accurately predict system behavior. In this domain, optical components act as waveguides rather than lenses.
2. Any simulation requires accurate material data to determine how EM radiation interacts with a given medium. Zemax OpticStudio includes data for many materials, mainly glasses and plastics that transmit in the visible, infrared and ultraviolet regions of the EM spectrum. All of these materials in the glass catalog are defined with a minimum and maximum wavelength. These represent the spectrum of wavelengths over which the data are considered accurate. When modeling any system, care must be taken to ensure the accuracy of dispersion and thermal data. This is especially true when working with wavelengths far from the visible region. The same consideration should be applied to materials used in coatings.

If neither of these assumptions are violated, then OpticStudio can be used to model any portion of the electromagnetic spectrum for a given optical system.